you must by law have a front plate, and it must be as vertical as possible, i.e not slopping on the bonnet or in the drivers window
road vehicle regulation (Display of Registration Marks) 2001
Fixing of front registration plates: vehicles registered on or after the relevant date
6. - (1) This regulation applies to vehicles, other than works trucks, road rollers and agricultural machines, first registered on or after the relevant date.
(2) Except as provided in paragraph (5), a registration plate must be fixed on the front of the vehicle in the manner required by paragraph (3).
(3) This paragraph requires the plate to be fixed -
(a) vertically or, where that is not reasonably practicable, in a position as close to the vertical as is reasonably practicable,
(b) in such a position that in normal daylight the characters of the registration mark are easily distinguishable from every part of a relevant area having the diagonal length specified in paragraph (4).
